To provide a hot water heat accumulator accumulating heat of surplus steam generated by gas turbine facilities with the form of usable hot water without waste.

A hot water heat accumulator is structured as follows. It is provided with a hot water cistern 12 storing hot water and a hot water generator 18 at the outer part of the hot water cistern 12. Lower temperature hot water is conducted from the lower part of the hot water cistern 12 by the hot water generator 18 and fed to the hot water cistern 12 heating the hot water mixing it with steam from the outside. A hot water taking out path 45 for taking out the hot water and feeding it to hot water using part is provided.